During the conventional coating of sheet metal small surface areas are sometimes inadvertently allowed to remain uncoated. That is, the coating material, such as enamel, fails to cover the entire sheet leaving small areas of the metal'surface exposed to the atmosphere. Left undetected, such bare spots cause serious problems as rust and other undesirable chemical reactions will result. The canning industry is an example of an activity that cannot afford to make its product from defectively coated sheet metal. Sheets which have bare spots must be detected and removed before fabrication.
Efforts have been made to detect such bare spots. The results have been disappointing. One attempt has been to use electrically charged free-end terminals which contact the coated sheet metal as it moves under the terminals. Proximate terminals carry opposite electrical charges. A current does not normally flow. However, when a bare spot exists between the terminals it causes the sheet metal itself to become a conductor between the terminals. The complete circuit activates means to stop the fabrication machinery so the defective sheet may be removed.
Unfortunately, with protracted use the terminals become bent out of shape and no longer contact the sheet metal. Also, they can only be used on one side of sheets which are often coated on both sides. Until my invention it has been necessary to check each coated side, one after another, rather than both simultaneously.